The present invention relates to light emission from semiconductor junctions in general, and in particular when those junctions are operated in the avalanche mode, as the active regions of Avalanche Light Emitting Diodes (ALEDs), thereby enabling light emission from indirect bandgap materials. It relates to the design of device layers and layout, as well as the method of fabrication, suitable for monolithic integration of ALEDs with sub-micron and sub-100 nm CMOS technologies, forming “Light Emitting Elements” (hereafter referred to as LIXELs). LIXELs can be implemented with Silicon bulk substrates, Thick-Film SOI substrates, or ultra-thin-film Silicon-On-Insulator (SOI) substrates, as well as with Germanium bulk substrates or ultra-thin-film Germanium-On-Insulator (GOI) substrates. Thin-Film GOI substrates are good candidates to be the used for sub-45 nm CMOS technology.
In the early years of semiconductor technology, it was noticed that silicon junctions operated in the avalanche mode emit white light. In fact it seems that light emission takes place across a large region of the electromagnetic spectrum, from the Long Wavelength Infra-Red (LWIR) to the Ultra-Violet (UV). Such wide interval of photon energies is an indication that different physical mechanisms, with different probabilities and efficiencies, are responsible for the emission of photons. Recent reviews of this topic can be found in: N. Akil, S. E. Kerns, D. V. Kerns, Jr., A. Hoffmann, J.-P. Charles, “A Multimechanism Model for Photon Generation by Silicon Junctions in Avalanche Breakdown”, IEEE Trans. on Elect. Dev., Vol. 46, No. 5, May 1999, pp. 1022-1028, and M. de la Bardonnie, D. Jiang, S. E. Kerns, D. V. Kerns, Jr., P. Mialhe, J.-P. Charles, A. Hoffman, “On the Aging of Avalanche Light Emission from Silicon Junctions”, IEEE Trans. on Elect. Dev., Vol. 46, No. 6, June 1999, pp. 1234-1239.
It is thought that some of those mechanisms are: (1) Interband transitions between (1a) hot electrons and thermal holes, (1b) hot holes and thermal electrons, (1c) hot electrons and hot holes; (2) Intraband transitions, (2a) in the conduction band, and/or (2b) in the valence band; (3) Brehmstrahlung due to scattering of hot carriers by ionized impurities.
Even though there has been ample experimental evidence since the 1950's that silicon can emit light, the efficiency has always been very low: roughly only 1 in 107 recombinations across the bandgap emit light. This low efficiency is tied to the details of the band structure of silicon, namely the smallest bandgap is indirect at 1.1 eV, to device design/geometry, and to process architecture.
Conventional avalanche light emitting devices are made by ion implantation into a bulk substrate, to make either lateral or vertical pn-junctions. In either case, the location which emits light can be hundreds of nanometers away from the substrate surface, and consequently photons with energy larger than the minimum bandgap of the substrate are absorbed, thereby severely reducing the external power efficiency.
For all the reasons mentioned above, band-structure, device design, and process architecture, it has been impossible to take advantage for practical applications, of light emission from silicon junctions operated in the avalanche mode. On the other hand, conventional CMOS technology is not amenable to the integration of other semiconductor materials for the purpose of bandgap engineering of pn-junctions. For this reason full monolithic integration of efficient light emitting devices with CMOS has not been possible.
The present invention, based on the device and process architectures disclosed in WO 2002/33755, and in WO 2004/027879, and the new layout designs disclosed in a co-pending application, presents a new method of fabrication, device layers, and layout designs that enable the monolithic integration of ALEDs with advanced CMOS, including sub-100 nm technologies, in which the light emitting regions can be made of materials other than the semiconductor substrate (e.g., silicon or germanium). It also discloses optimized doping and heterojunction profiles for the purpose of increased efficiency of light emission, as well as optimized profiles for the purpose of light emission in certain ranges of wavelengths, namely in the 1.3 μm and 1.55 μm ranges.
The monolithic integration of ALEDs with advanced CMOS technology, in one exemplary implementation, requires only three additional masks, with respect to the number of masks required for the CMOS technology in question. It has been experimentally verified that the avalanche photo-diodes described in WO 2002/33755 and WO 2004/027879, with one of the layouts described in co-pending application, do emit light under certain conditions of operation.